The Urinary System Components 2 kidneys 2 ureters 1 urinary bladder 1 urethra Primary function is homeostasis of both the composition and the volume of blood This is accomplished by removing most of the water and solutes from the blood and then selectively restoring them Unneeded H2O ions solutes and toxins are then excreted in the form of urine Other Functions 1 Regulates blood pressure by producing a hormone enzyme renin Causes water retention increased blood volume and vasoconstriction constricts blood vessels 2 Producing erythropoietin which stimulates erythropoiesis 3 Regulates blood pH acidity 4 Activates Vitamin D Nephron the functional unit of a kidney There are over 1 million of these blood processing units which filter blood and then selectively reabsorb the filtrate or allow it to become urine urine is what is not reabsorbed A nephron consists of a renal tubule enclosing a capillary network called a glomerulus The end of the renal tubule expands and encloses the glomerulus and is called the glomerular capsule o Collectively the glomerulus and it s capsule are called a renal corpuscle The capillaries of the glomerulus are fenestrated very porous These pores allow virtually all the blood s plasma except large proteins to leave the capillaries and filter into the glomerular capsule The visceral layer of the glomerular capsule the layer touching the glomerulus is composed of branching octopus like epithelial cells called podocytes The arms of the podocytes wrap around the fenestrated capillaries leaving openings called filtration slits they control the opening to the pores in the capillaries Physiology The work of the urinary system is done by nephrons all the other structures are simply for storage or passageway The system filters about 45 gallons of blood day The plasma is filtered removed and returned 60 times a day 3 Steps in Urine Formation 1 Glomerular Filtration The plasma and solutes are forced out of the glomerular capillaries by hydrostatic pressure blood pressure This essentially non selective process involves the plasma being forced through A The fenestrated capillary endothelium B A basement membrane C The filtration slits left by the podocytes of the visceral glomerular capsule The pressures involved in glomerular filtration are A Glomerular blood hydrostatic pressure GHP Blood pressure entering glomerulus This is kept high because the afferent arteriole incoming vessel is much bigger than the efferent arteriole This pressure is about 60mm Hg mercury and forces plasma into the glomerular capsule And there are 2 opposing forces B Capsular hydrostatic pressure CHP Resistance due to fluids already in the glomerular capsule Amounts to 20mm Hg C Blood osmotic pressure BOP remember osmosis is diffusion of water through a permeable membrane from a dilute area toward a more concentrated one Since the blood remaining in the glomerulus is more concentrated than the filtrate the water tries to move back into the glomerular capillaries creating a resistant pressure of about 30mm Hg N F P Net Filtration Pressure the net result of the pressures NFP GHP CHP BOP 60mm 20mm 30mm 10mm It is important how they cancel each other out leaving little pressure for filtering If filtration stops toxins are left in the blood which is deadly renal failure can result from a drop in blood pressure GFR Glomerular Filtration Rate the amount of filtrate flowing out of all the renal corpuscles of both kidneys per minute Normal 125ml min 180 L 48 gallons of filtrate day 99 of this filtrate is reabsorbed back into the peritubular capillaries leaving 1 5 liters of urine day The return of filtrate to the blood stream is called Tubular reabsorption 2 Tubular Reabsorption Reabsorption occurs both passively diffusion and actively active as in requiring ATP and a carrier mechanism from the tubule back into the blood stream Passive reabsorption is essentially diffusion of water and small solutes ions 80 of H2O in the filtrate diffuses back into the peritubular capillaries we can increase H2O reabsorption by ADH antidiuretic hormone which increases the permeability i e porosity of the distal end and collecting renal tubules an increase in permeability increases reabsorption of H2O thus blood volume and pressure increases Active reabsorption is the way larger solutes such as glucose or amino acids are recovered Because it involves ATP and carrier mechanisms the body an control what is reabsorbed Some things aren t reabsorbed either because they are too big or because there is no carrier mechanism for them this would include most drugs and toxins if they were small enough to pass through the filtration membrane in the first place Tubular reabsorption is from the renal tubule back into the peritubular capillaries Chemicals still in the blood which are not needed are discharged into the renal tubule by 3 Tubular Secretion an active process important in A Eliminating substances too large to pass through the pores slits B Eliminating undesirable or excess substances which were passively reabsorbed C Ridding the body of potassium ions D Controlling pH Each nephron has a region called the Juxtaglomerular apparatus JGA This is where the distal convoluted tubule touches the afferent and efferent arterioles Specialized cells in the JGA regulate the GFR filtration rate They monitor filtrate flow and if the GFR is too low they release renin If filtration rate is too low it is because blood pressure is too low Renin causes 1 vasoconstriction and 2 the release of ADH both of which raise blood pressure If the FGR is too high the nervous system constricts the afferent arteriole which lowers GFR Kidneys retroperitoneal organs located behind the parietal peritoneum found right below the 12th thoracic vertebrae they are held in place and protected by 3 tissue layers Renal Fascia Adipose Capsule Renal Capsule Connective tissue sheet anchors kidney to body wall Beneath the fascia it holds kidney to fascia and cushions it against trauma Transparent innermost layer is a barrier to infection The ureters run retroperitoneally about 12 to the bladder and they enter the bladder from below it This arrangement means that during the pressure of urination the ureters are compressed and closed preventing backflow of urine to the kidney o This prevents the spread of relatively common bladder infections up to the kidneys Both the ureters and the bladder are lined with alkaline mucus secreting mucosa Beneath this is a muscularis then a
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